Hollow metallic shafts are ubiquitous in our society. They are used in products where lightness and strength are required. Products which utilize such hollow shafts include the more esoteric, such as sporting goods, to the more utilitarian, eg. cantilever street light poles and furniture of all types. With respect to sporting goods, they may be used for golf clubs, bicycle frames, ski poles or for the vertical supporting members of volleyball nets.
The materials presently used to construct hollow metallic shafts include the following: aluminum, low and high carbon steels, as well as stainless steels and coated steels, including, but not limited to galvanized steel, composites and graphite. However, these materials have limitations and may be objectionable for numerous reasons, including, but not limited to, weight, rigidity, flexibility and, in their use in sporting goods, torque resistance. What is needed is a hollow shaft which is light, strong and flexible.
Shafts which are presently used with sporting goods comprise metallic as well as composites and graphite materials. While composite and graphite shafts are light and are strong, they have minimal flexibility so they tend to snap. Aluminum is light, but lacks the strength of steel. Steel is usually too heavy and corrosive. Indeed, the increased use of composites and aluminum clearly suggest that the use of steel is inappropriate.
Lightness, strength, and flexibility are not the only characteristics that one desires in a hollow shaft used in a manner suggested above. An additional desirable characteristic would be if these shafts are corrosion resistant, thereby extending the lifetime of the products made with these shafts. Graphite, composites and stainless steels are all effective in their resistance to corrosion.
With respect to the steels, the use of alloy steels, in particular stainless steels, have been utilized in the manufacture of hollow shafts. The stainless steels comprise the ferritic, martensitic and austenitic structures. Of these, the austenitic stainless steel is the most widely used, this steel being characterized by its high nickel content and hence its being non-magnetic. The austenitic stainless steels are further characterized as essentially non-hardenable through the application of heat. Only martensitic stainless steel is hardenable by heat treating. Stainless had heretofore been the choice in many products where the use of a shaft that was light, flexible and strong. However, there was no ability to fashion the stainless steel to provide strength where needed, yet eliminate the weight where such strength was not needed. The present invention solves this problem.
With respect to the use of metal shafts for sporting goods, the prior art suggests numerous, but unsatisfactory, remedies of these problems: reinforcing tubular shafts with compressible tubular materials having a plurality of reentrant portions extending longitudinally and separated by longitudinally extending ribs, filling the hollow shaft with plastic materials or rubber, etc. The present invention provides an improved hollow shaft which can be crafted so as to provide the strength in the appropriate locations, whether the shaft is to be load bearing or used to strike a projectile.